The use of intermediate frequencies (IF) in radio frequency (RF) transmitters and receivers is known. Typically an IF frequency is of a relatively constant frequency that varies only by a modulation level of an impressed information signal. In the case of a transmitter, the modulated IF frequency is shifted upwards to a final channel frequency by mixing the IF frequency with a variable frequency from a frequency synthesizer.
Philosophically, the greater the difference between the IF frequency and the final channel frequency, the easier it becomes to filter an information signal from other, unwanted artifacts. When an IF frequency is mixed with the final channel frequency, the result is a summation frequency and a difference frequency and multiples thereof (aliases). Most of the information energy lies in the summation and difference signals and typically one (or both) are amplified for transmission to a distant receiver. The aliases represent interference, both to the information signal and to information signals on adjacent channels and, consequently, must be filtered from the information signal before transmission.
When the difference between the IF frequency and final channel frequency is great, the difficulty of filtering the aliases from the information signal becomes much easier. As a consequence, an IF frequency is typically chosen to maximize the frequency difference between the IF frequency and any frequency the synthesizer may assume in transmitting on an assigned channel.
The information signal translated to an IF frequency may be analog or digital. Historically, the information signal has been analog, originating from such sources as microphones. Where the information signal is analog, the technology used for translation of the information signal to the IF frequency has included the use of mixers and filters.
More recent systems have used digital processing. Analog to digital converter (A/D) converters and digital signal processors (DSP) may be used to transform analog signals (e.g., from a microphone) into a digital format. The conversion to a digital format has also allowed computers to be used as sources of an information signal transferred over a radio frequency link.
The availability of digital signal processing has resulted in some simplification of signal processing hardware. For example, techniques have been developed which allow the generation of transmitted waveforms from a digital information source using waveform lookup tables. Multiplexers may be used to combine waveforms from the waveform tables to generate combinations of waveforms to simulate a variety of modulation levels.
Other techniques include interpolating digital signals and delta-sigma modulating an input driving digital to analog (D/A) converters to generate an IF waveform. Bandpass filters may be used to control aliasing of the digitally converted products.
In general, the generation of the IF signal inevitably involves modulation of a source information signal onto an IF reference frequency. IF exciters have been developed which rely on various mixing philosophies and phase change algorithms. While such methods have generally been effective, their effectiveness is dependent upon extensive filtering and the use of analog components having relatively precise tolerances. The introduction of digital techniques have generally not changed these requirements. Because of the importance of radio frequency communications, a need exists for a method of IF excitation that is not dependent upon the precise tolerances of analog signal generation and extensive filtering.